Pyroelectric elements have been used as infrared detectors for security and gas detection purposes. A pyroelectric element includes a pyroelectric substrate and a pair of electrodes disposed on front and back sides of the pyroelectric substrate. Irradiating a surface of the pyroelectric element with infrared light increases the temperature of the pyroelectric substrate. The change in temperature changes the spontaneous polarization, disturbs the balance of electric charge on the surface of the pyroelectric substrate, and causes generation of electric charge. The infrared light is detected by extracting the generated electric charge through conductive lines connected to the pair of electrodes.
A pyroelectric element is known which includes a pyroelectric substrate made of lithium tantalate (LiTaO3, hereinafter may be abbreviated as “LT”) that has a large pyroelectric coefficient and a high performance index (see, e.g., Patent Document 1). The pyroelectric substrate made of LT suffers the problem of occurrence of sudden noise called popcorn noise. The popcorn noise is considered to be caused by discharge of electric charge which is accumulated on the surface of the pyroelectric substrate and increased in voltage to a high level. Some techniques have been developed to suppress the occurrence of popcorn noise (see, e.g., Patent Documents 2 and 3). Patent Document 2 describes a technique in which the surrounding of a light-receiving section is removed on three sides to suppress stress concentration caused by application of thermal change, and thus the occurrence of popcorn noise can be significantly reduced. Patent Document 3 describes a technique in which when the volume resistivity of a pyroelectric substrate is set to 1×1010 Ωcm to 1×1013 Ωcm, since the voltage on the surface of the pyroelectric substrate is not increased to a high level and the occurrence of discharge can be suppressed, the occurrence of popcorn noise can be reduced. This is because even if a change in temperature causes generation of electric charge, the generated electric charge passes through the pyroelectric substrate and disappears quickly.                Patent Document 1: Japanese Unexamined Patent Application Publication No. 6-160194        Patent Document 2: Japanese Unexamined Patent Application Publication No. 10-2793        Patent Document 3: Japanese Unexamined Patent Application Publication No. 2006-203009        